Modern telecommunication technologies based on optical fibers and associated optoelectronic components are designed to deliver a variety of advanced services ranging from ultra-long distance submarine transmission, to very high capacity 10 Gbs and 40 Gb/s long-haul terrestrial backbone, through Metropolitan Area Networks (MANs) and access networks such as the “triple play” subscriber access of plain-old-telephone-service (POTS), broadband Internet, and television. The penetration of such “triple-play” service by provision of optical access networks with projected subscriber terminations requiring massive quantities of optical line terminal (OLT) equipment is strongly driven by the operators and service providers cost factors in growing and maintaining their subscriber base in a highly competitive environment. Such economic drivers including lowering overall cost-of-ownership, increased services and service delivery, and resulting in continued downward pressure on the core optoelectronic componentry. This, in turn, forces the component vendors to look for more inexpensive, smart, efficient and multi-functional approaches to providing these components and functionalities.
In this regard, semiconductor based photonic integrated circuits (PICs) in which several functions such as optical signal detection, modulation and optical signal emission are implemented in a single monolithic semiconductor chip appear to be a promising solution. Further indium phosphide (InP) and its related III-V semiconductor material system offer additional benefits as they allow the fabrication of active devices operating in the important wavelength ranges around 1300 nm and 1550 nm, i.e. in the two dominant low-loss transmission windows of the glass fibers. However, even such monolithic integration can provide cost barriers with poor design methodologies, low manufacturing yields, complicated manufacturing processes and repeated expensive epitaxial growth processes. Accordingly a single step epitaxial wafer growth methodology, in conjunction with established wafer fabrication technologies, can provide a means to further enable reduced optical components cost.
The integration not only of multiple optical functions on one chip, but multiple functions operating at multiple wavelengths, relies on a careful design and implementation of the epitaxial structure grown onto the substrate wafer and into which specific functions are implemented with specific association with a particular layer in the structure. Providing optical coupling from one layer to another, as well as controlling the spatial field profile of propagating optical signals are inevitable challenges in the PIC design. Within the prior art it is a general requirement for semiconductor lasers and other optoelectronic devices to operate in a single fundamental mode regime, which is approaching optimal from controllability point of view. Involvement of higher-order modes is commonly considered as a detrimental factor and usually special care is taken to suppress higher-order modes and to guarantee stable fundamental mode operation. However, in some circumstances operation in higher order lateral mode appears to be beneficial over fundamental mode. For example, optical couplers or other distributed feedback active or passive devices exploiting lateral grating operate more efficiently in higher order mode because higher lateral mode have better overlap with the grating. In this situation, in order to provide interface with components operating in fundamental mode (including single-mode optical fiber) lateral mode converter is necessary.
To generate high power, the optical cavities of semiconductor lasers are often required to be sufficiently wide enough that in addition to fundamental transverse mode another higher order mode comes into play. This happens in high power GaN-based lasers with wide laser ridge as demonstrated by Swietlik et al (“Mode Dynamics of High Power (InAl)GaN Based Laser Diodes Grown on Bulk GaN Substrate”, Journal of Applied Physics, Vol. 101, P. 083109, 2007), and AlGaAs broad area lasers studied by S. Blaaber et al (“Structure, Stability, and Spectra of Lateral Modes of a Broad-Area Semiconductor Laser”, IEEE Journal of Quantum Electronics, Vol. 43, p. 959, 2007).
Another case is laterally coupled distributed-feedback lasers where the first order lateral mode can provide a coupling efficiency superior to the fundamental zero-order mode, see for example Matsuda et al (“Optical Device Coupling Light Propagating in Optical Waveguide with Diffraction Grating”, US Patent Application 2007/0133648). However, whilst efficient and stable optical generation is achieved in the first or in other higher-order mode, the challenge is then an efficient transfer of the optical signal from the spatial profile of the higher-order mode into the fundamental mode of an optical fiber or another photonic component.
Therefore, it would be highly desirable to develop a mode converter, or mode transformer, which may be easily integrated with an semiconductor laser on the same PIC whilst allowing efficient conversion of higher-order mode light signal into a field having good matching with the fundamental mode of optical fiber. In addition, it would be advantageous if the whole structure, laser and mode converter, supported preferentially the generation of the selected higher-order mode in a positive way, e.g., by suppressing all undesired modes and stabilizing the desired higher-order mode.
PIC technology which assumes integration of several functions within the same optical circuit usually relies on exploiting a multilayer waveguide structure. From a cost point of view it is highly advantageous to produce such a multilayer structure during a single epitaxial growth step, thereby avoiding multiple re-growths, and to form active and passive waveguides on different levels of the structure by standard photolithography and etching techniques. In this case, the mode transformer design should be compatible with general functioning of the multiple vertical waveguide photonic circuit. Namely, the mode transformer should convert higher-order mode generated in the laser waveguide on one structure level into the fundamental mode of the passive waveguide on another vertically separated level. In addition, the optical field in the fundamental mode should be suitable for coupling either into optical fiber or into another passive or active component integrated on the same chip.
Several versions of mode transformers are described in the prior art. For example, Van der Tol (“Optical Switching Device”, U.S. Pat. No. 5,574,808) proposed an optical device which incorporates a passive mode converter for transforming one guided mode into another by means of periodic coupling between the first and second guided modes in a bimodal channel-type waveguide. The approach being based on the fact that in a bimodal channel-type waveguide, the field profiles for two modes are characteristically different and, therefore, one of the profiles can be more strongly modified by creating external perturbation to the refractive index profile. The desired index perturbations were proposed to be achieved by means of electro-optical or thermo-optical effects which make the mode-converter quite sophisticated from both fabrication and operational points of view. Another limitation of the invention is that it converts an initial transverse electric (TE) zero-order mode into a transverse magnetic (TM) first-order mode as is disclosed in the patent, conversion of the mode of higher order being taught as problematic. It would be highly desired to develop a mode transformer able to convert modes of arbitrary order and free from any need for electro-optical control.
Another prior approach is that of Kazarinov et al (“Optical Waveguide Multimode to Single Mode Transformer”, U.S. Pat. No. 6,580,850) wherein there is taught a waveguide-based mode transformer to facilitate coupling from a wide-area semiconductor laser diode into a single mode optical fiber. The waveguide transformer reliant upon using a separate planar waveguide component which accepts light from the laser chip. Subsequently, the light received by the transformer is supposed to be filtered in a way that higher order modes were “stripped off”, leaving only portion of the light corresponding to the laser fundamental mode. Another embodiment of the prior art of Kazarinov transforms a first-order laser mode into a passive waveguide fundamental mode by coupling two lobes of the first-order mode into two separate waveguides and using a planar Mach-Zehnder interferometer to subsequently combine the light fields from two waveguides into one. The fact that two separate chips are assumed to be used for light generation and for mode conversion makes the invention not suitable for advanced PIC devices intended for monolithic integration. Namely, an enormous difficulty related to the light coupling from one chip to another and high insertion loss associated with such coupling rules out application of the prior art by Kazarinov in the monolithic integration architecture.
Recently Vorobeichik et al (“Method and Apparatus for Optical Mode Conversion”, U.S. Pat. No. 7,218,814) disclosed a waveguide structure for conversion of one mode of the input waveguide into another mode of the output waveguide. As with the prior art of Van der Tol this mode conversion relies on creating perturbations in the refractive index profile of the waveguide by applying a controlling signal to appropriately positioned electrodes on top of the mode conversion waveguide. However, the electrodes are arranged in the form of the grating whose period must be reciprocal to a phase difference between the first guided mode and the second guided mode. Since the refractive index grating is created through thermo-optic effects, the invention suffers from the drawbacks specific to the devices requiring resistive heating. Namely, sophisticated electrical circuitry and packaging, device calibration and re-calibration when external conditions change, and an overall increase in both device production and operation costs.
Another prior art approach suggested by Leuthold et al (“Multimode Interference Couplers for the Conversion and Combining of Zero- and First-Order Modes”, IEEE Journal of Lightwave Technology, Vol. 16, No. 7, pp. 1228-1239, 1998) is based on multimode interference wherein modes of different orders can transform one into another after propagation in a properly designed multimode waveguide. While the suggested approach can be readily used for monolithic integration of the mode converter into a photonic chip, demonstrated performance of conversion efficiency is low and not compatible with the constraints of an optical power budget within the PIC to meet commercial OLT specifications at neither low cost nor optical crosstalk requirements between the multiple elements of the PIC.
Currently PIC architectures are based on multi-layer epitaxial structures where a specific function, such as light generation, multiplexing or detection, is implemented at a particular waveguide layer. From this perspective, mode conversion should be compatible with other functions implemented at any of the particular passive waveguide levels allowing flexibility in multiple emitters and the wavelengths of the emitters. For example, 155 Mb/s BIDI transceivers, wherein modules are required as both 1310 nm emitter/1550 nm detector and 1310 detector/1550 nm emitter. In the prior art developments of multiple waveguide structures are limited, see for example Suematsu et al (“Integrated Twin-Guide AlGaAs Laser with Multiheterostructure”, IEEE J. Quantum Electron., Vol. 11, pp. 457-460, 1975) wherein only two vertically-separated waveguides, one active and one passive, are coupled via resonant optical coupling between them. The requirement for increased integration and more advanced functionality has resulted in the establishment of the multiple guide vertical integration platform proposed by Tolstikhin et al (“Integrated Vertical Wavelength (De)Multiplexer” U.S. patent application Ser. No. 11/882,126). Beneficially the incorporation of a mode converter into the multi-guide vertical integration platform would extend functionality of the latter.
Therefore, it would be highly advantageous to provide a solution for the mode transformer that removes the constraints of the prior art and enhances the functionality of existing PIC architectures. It would be further advantageous if the solution was compatible with a single epitaxial growth and standard semiconductor fabrication processes allowing relaxed fabrication tolerances.